The invention pertains to a method for obtaining information on the residual errors in a transmitted channel-decoded digital signal, especially in respect to the transmission of images over mobile radio channels.
Not only block encoding but also "folding" encoding are used to protect the transmission of signals (video, audio, data) from error. One of the standard ways of decoding folding encoding is based on the so-called "maximum likelihood criterion", according to which the encoded word which is decoded is the one most likely to have been transmitted. The associated decoding instructions are efficiently carried out by means of the Viterbi algorithm. When a so-called "soft" decision is used for decoding, not only the binary symbols but also certain "reliability information" is used in the decoding process. As a result, a gain can be achieved with respect to the required signal-to-noise ratio on the channel. On the basis of the Viterbi algorithm, methods have become known which make it possible to obtain reliability information pertaining to the channel-decoded symbols. This reliability information can be used advantageously, for example, for error masking (as described in DE 41 37 609 A1), for adaptive error correction, or for chained encoding. It represents a measure of the instantaneous transmission error. There is no point in making direct use of the reliability information obtained in the form of symbols, however, because the probability that a symbol has been falsely decoded is less than 0.5. A Viterbi algorithm with soft-decision output signals and its applications are described in J. Hagenauer and P. Hoeher: "A Viterbi algorithm with soft-decision outputs and its applications", Proc. IEEE GLOBECOM '90, Dallas, Tex., pp. 47.1.1-47.1.7, November 1989.